DISSECTION OF THE DOG 169 



moveable partition between the mouth and the pharynx. One border is 

 attached to the margin of the bony palate ; while the other border, which forms 

 the dorsal boundary of the isthmus faucium (the opening from the mouth into 

 the pharynx), is free, and, in a state of rest, is in contact with the epiglottis of 

 the larynx. Of the two surfaces, the dorsal is continuous with the floor of the 

 nasal cavity and is slightly convex. The ventral surface, which continues the 

 hard palate, is correspondingly concave. 



Prolongations of the mucous membrane of the soft palate pass to the tongue 

 and the pharynx in the form of folds known as the glosso-palatine and pharyngo- 

 palatine arches (arcus glosso-palatinus et arcus pharyngo-palatinus). The 

 pharyngo-palatine arch really consists of two diverging folds, the more dorsal 

 of which is the better developed and ends on the dorsal wall of the pharynx. 

 The other fold of the arch passes to the epiglottis. 



Between the glosso-palatine and pharyngo-palatine arches is a deep depres- 

 sion, the tonsillar, sinus (sinus tonsillaris), bounded by prominent folds of 

 mucous membrane, and containing the palatine tonsil (tonsilla palatina), an 

 elongated eminence composed mainly of lymphoid tissue. Immediately 

 dorsal to the tonsillar sinus there is a shallow supratonsillar fossa (fossa 

 supratonsillaris) . 



Structurally the soft palate may be described as composed of four layers : 

 (1) Mucous membrane continuous with that of the hard palate ; (2) a layer of 

 glandular tissue, thickest towards the free border of the palate ; (3) an aponeu- 

 rotic and muscular layer containing the pharyngo-palatine muscles and the 

 muscles of the uvula ; (4) mucous membrane continuous with that of the nasal 

 cavity. Naturally the mucous membrane on the two surfaces is continuous 

 round the free border of the palate. The pharyngo-palatine muscle has 

 already been examined. 



M. vnvlm. — The muscle of the uvula is a rounded strip running from the 

 margin of the palatine bone to the free edge of the soft palate. The muscles 

 of the two sides are close to each other and are generally blended at their 

 termination. 



Mouth. — The mouth is the initial part of the alimentary canal and occupies 

 the more ventral part of the face. Its cavity comprises two parts : (1) a 

 vestibule (vestibulum oris) and (2) a proper cavity (cavum oris proprium). 



The vestibule of the mouth consists of a narrow space between the lips and 

 the cheeks on the one hand and the teeth and the gums on the other. The 

 mucous membrane reflected from the lips and cheeks to the maxillary and 

 mandibular bones forms its upper and lower boundaries. In front and to each 

 side the vestibule communicates with the exterior by an elongated fissure-like 

 opening (rima oris) between the lips. Communication between the mouth 

 cavity proper and the vestibule is effected by irregular openings between the 

 teeth of the opposed jaws and behind the last cheek-tooth. Into the vestibule, 

 on each side, open the ducts of the parotid and buccal glands. 



